Depending on how much "extra" you need - I did a small quilt with only one inch extra - so for your quilt, I would have had 46 x 61 - but that is cutting it waaaaay too close.
If it is going to be long-armed, you probably will need to have something at least 53 x 68 (4 inches extra on each side)
If it were mine, I would cut the long length into 53 inch sections (or whatever length you need) , and sew them together, then cut off the excess. The seam would go across the short length of the quilt and would be off-center (which I think is preferable to a center seam, anyway).
I would straighten one of the ends before starting to cut - either by tearing (cringe) or by pulling a thread - and then measuring from the straightened point.